The one time i heared the fostex, it was very bright, piercing in the ear. It could be due to the backloaded horn it was in, but i did not like it. They need a filter circuit to sound good for me.
The Alpair is much more smoother, goes lower and more detailed. I've build some 14L ported speakers for someone with it tuned to 50Hz without filter and it sounded very good for a bookshelf. But there are many options and existing designs for it...
The Apair 7 has a smoothr response, is much more versatile in terms of box possibilities, more bass potential, but all at the price of efficiency. The efficiency comes with greater issues with the cone ringing. Fostex has something often called FOstex shout, some peakiness in the upper ranges that drive some crazy and doesn’t bother others. I am currently having a converstaion with a member whio is over the moon with his FE126 in FH1 (which is designated surpassed by FH3). He listens mostly to classical music.
The Fostex FExx6En series all want a higher output impedance amplifier. The Alpair is more agnostic to amplifiers.
The Fostex really wants to be in a horn. Valiant, Austin A126, Kongo, and FH3 are the leading candidates.
The Alpair 7 fits intoMaeshowe, and FH3, but there are also a large range of proven reflex and transmission lines that work well.
Both drivers can be improved, but there are many more steps with the Fostex.
The Alpair 7.3eN (EnABLed current model) is my #1 favorite driver. It is our goto choice for a midTweeter in our biggest WAWs. The new Pluvia 7HD is touted as a development of teh A7.3, the Alpair 7ms is the new no spider driver the same size. It has a lot of promise (i have both, haven’t heard them yet). The Alpair 7 MAOP is a special version of the A7 that is very rare, and essentially unobtainium.